(1) Field of the Invention
This invention generally relates to a control system located at a first, or reference, site for guiding a steerable object from that site toward a second, or target, site. More specifically this invention relates to such a control system that is operable even when both the control system at the first site and the target at the second site undergo independent motion.
(2) Description of the Prior Art
Real time control systems based upon sensory inputs find application in air-, land- and underwater-based vehicles. For example, with respect to underwater-based applications U.S. Pat. No. 4,323,025 to Fisher et al. (1982) describes a homing torpedo with an on-board, or autonomous, steering control system that operates in a target search phase or a target acquisition phase. In the target search phase, the control system directs the torpedo along a controlled helical search path. When the torpedo acquires a target, the control system transfers to a target pursuit phase. The control system transfers back to the target search phase whenever target acquisition is lost. It is a characteristic of control system of this type, however, that control from the submarine as a launching vehicle is lost immediately upon launch.
U.S. Pat. No. 5,229,541 to Will et al. (1993) discloses a variant of the foregoing on-board, or autonomous, control system in the form of a safety system that deters a homing missile from attacking its launching vehicle. In this embodiment, a tracking circuit on the launching vehicle tracks a torpedo after launch by receiving signals from a transponder on the torpedo. If the control system on the launching vehicle determines that the torpedo has reentered a protective zone on a course homing on the launching vehicle, the control system generates a coded command that it transmits to the torpedo control system acoustically. The torpedo control system responds by altering its course and resuming a search for a target in a sector other than that in which it was acquiring a launching vehicle. In addition upon determining that the weapon is within an activation zone, the control system on the launching vehicle can produce a magnetic field substantially corresponding to the neutralization zone. When sensors on the torpedo enter that field, they neutralize the weapon detonator.
Some torpedoes and other steerable objects include acoustic or other sensory homing systems. Such homing systems have an external point in front of and along the path of the steerable object called a "guidance point". This guidance point corresponds to the centroid of the acoustic beam in the case of a torpedo with an acoustic homing device. Control systems generally must accommodate torpedoes with or without guidance points. Typically, therefore, control systems use different control modes for guiding different torpedoes toward targets. In each, the control system resides on board the submarine that constitutes a first or reference site or launching vehicle. Each control system transfers commands to and receives information from the torpedo, as a steerable object, by means of a communications link, such as a wire link.
If the range, course, speed and bearing of a target are known, a "target intercept" control mode can be used. In the target intercept control mode, a control system predicts the trajectory of the target and directs the torpedo to an anticipated intercept point. A control system operating in a "target pursuit" mode directs the torpedo so that it always points toward the target. In a "beam rider" control mode the control system directs the torpedo along a bearing between the submarine and the target.
U.S. Pat. No. 5,319,556 to Bessacini (1994) discloses adaptive trajectory apparatus for selecting one of these control modes based upon information available during each update cycle for a given situation. Notwithstanding the selected control mode, once the torpedo comes within an effective range of the target, internal torpedo guidance assumes control.
Still other approaches for directing a steerable object from a launch site to a target involve complicated control systems. These systems are generally based on sets of differential equations and estimates of input parameters. Such systems operated in response to analytical controllers.
None of the control systems including those implemented in accordance with the foregoing Bessacini patent incorporate any mechanism for readily using heuristic information in establishing control, particularly information about expertise gained through past experience. Moreover, these control systems normally require an operator to determine whether to issue a particular command to a torpedo and do not automatically generate and issue guidance commands in a continuous fashion.
Our co-pending U.S. Pat. No. (application Ser. No. 08/498,811) discloses a beam rider control system for submarine launched torpedoes that utilizes a fuzzy controller at the submarine for generating the guidance commands transferred to the torpedo over the communications link. Such fuzzy controllers rely upon information from a torpedo model and a communications link to the torpedo. The torpedo model is a mathematical replica of the torpedo that provides position and status information for post launch guidance operation. The control mechanism utilizes measured contact information, particularly a bearing from the submarine to the target, and torpedo model information, particularly the bearing from the submarine to the guidance point of the torpedo, to generate a command sequence for maintaining the guidance point on a target bearing from the submarine to the target.
Such systems therefore have, as one goal, keeping the torpedo guidance point on the bearing from the launching vehicle to the target. However, the torpedo, as a source of noise, may interfere with or contaminate the signals from the target when it is on or proximate that bearing line. This interference can degrade sensed target information at the first site before the acoustic homing device in the torpedo acquires the target. Accordingly, in the system described in a co-pending application also operates with another goal of maintaining the torpedo at some distance from the bearing line from the submarine to the target. This control system, that can operate in an iterative fashion, selects one of several sets of predetermined sensed linguistic variables pertaining to first and second goal sets and the control system produces a corresponding control output linguistic variable based upon the selected goal.
This system also includes a conditioner that constrains all command information to prevent the torpedo from traveling in a direction with a searching velocity component directed back toward the submarine. This system also conditions the commands by controlling gain, but only when the control system produces a control output in response to the other goal, namely maintaining the torpedo guidance point on the bearing line. Thus if the control system is operating to remove the torpedo from proximity to the bearing line, the conditioner operates only in response to the constraint conditions. In the target intercept mode, the conditioner operates in response to both the gain adjustment and the constraint, with gain adjustment preceding the imposition of any constraint. The gain adjustment is a function of the distance from the torpedo guidance point to the target.
It has been found, however, that it is not a simple matter to transfer various aspects of hierarchical beam rider control systems to target intercept control systems. First, the target intercept and beam rider modes require different sensed variables. Second, the command information, gain adjustment and velocity constraint, if directly applied, adversely effect the resulting command and trajectory so that it becomes probable that the torpedo will not intercept the target.